Electron collector



Oct. 4, 1960 F. STERZER ELECTRON COLLECTOR Filed May 2, 1958 INVENTOR.

FRED ST ERZ ER United States Patent ELECTRON COLLECTOR Fred Sterzer, Monmouth Junction, N .J., assignor to Radio Corporation of America, a corporation of Delaware Filed May 2, 1958, Ser. No. 732,621

8 Claims. (Cl. 31'53.5)

This invention relates to electron discharge devices utilizing electron beams having a distribution of electron velocities and particularly to apparatus for collecting electrons from such a beam.

Certain present-day electron tubes, e.g., the velocity modulated beam type utilize such electron beams and may therefore incorporate the present invention. For this reason and for the sake of brevity, clarity, and convenience, this invention will be described as incorporated in a conventional traveling wave tube. It will be appreciated, however, that the invention is broadly applicable wherever eflicient collection of a multi-velocity electron beam is desired.

In the conventional traveling wave tube an electron beam traverses an RF delay electrode such as a conductive helix where it is velocity modulated, gives up energyin amplifying a Wave traveling along the helix, and is then collected by an electron collector. According to usual practices the electron collector comprises a single electrode which is maintained at a single potential equal to or only slightly lower than the potential of the helix. However, in the case of high power traveling wave tubes, electron collection according to such conventional practices becomes quite inefficient. To this .end, especially designed collectors can be more efficiently operated at potentials considerably depressed below the potential of the helix and, in fact, plural-stage collectors comprising a plurality of collector electrodes maintained at a corresponding plurality of different depressed voltages can be efliciently utilized.

In a conventional traveling wave tube most of the electrons in traversing the helix interact with the RF fields and are slowed down. The velocities of the electrons according to convention may be expressed in volts. If an electron has been decelerated by an amount corresponding to AV=V V where V =the voltage on the helix and V =the voltage of an electron entering the collector region, then it follows from conservation of energy that the lowest potential at which this electron can be collected is given by V =AV.

Thus, it can be seen that for any electron to be collected by a collector, the collector potential must be equal to or greater than the amount which the electron has been slowed down, or stated otherwise, it must be equal to or greater than the helix potential minus the electron potential.

Although some electrons in traversing the helix will be slowed down to such an extent that a collector potential at or near the helix potential will be necessary in order to collect them, other electrons will not be so slowed and will, in fact, emerge with higher velocities. These higher velocity electrons can, according to the electron collection theory expressed above, be collected at a potential lower than the potential necessary to collect the slower electrons. For this reason it becomes apparent that many of the electrons can be collected by potential. Extending this principle, and considering that a collector having a potential depressed below the helix "2,955,225 Patented Oct. 4, 1960 the electrons emerge from the helix having a distributionof velocities, a series of collectors-or a plural electrode collectorcan be employed to efliciently collect the various electrons at the lowest possible collector potentials.

According to the above-stated theory of electron collection, advantages of using depressed collector potentials can be seen. When an electron collector is operated with a single potential high enough to collect the slowest electrons, the higher velocity electrons, in being collected, Will give up their excess velocity energy to the collector in the form of heat. This heat must be dissipated by some means Such dissipation becomes a very important consideration in high power traveling wave tubes where the power necessary to operate the cooling means is often greater than the power dissipated in the collector. And even in medium power tubes, excessive collector heating often necessitates the resorting to liquid cooling rather than permitting the simpler air cooling.

The use of depressed voltage collectors also provides important advantages in the minimization of excessive X-ray radiation from the collector. This may mean substantial savings in the amount of shielding required around the tube. Depressed collector voltages also mean a lower D.C. power input requirement to the tube. Since the overall eificiency of the tube is equal to the RF power output divided by the D.C. power input, an increase of tube efficiency results.

However, when the collector is operated at considerably depressed potentials, several harmful effects can occur which are negligible if the collector is operated at a potential near that of the helix. -In the region between the depressed collector "andnhe'ihelix, an electrostatic field is set up which tends to draw the secondary electrons emitted by the collectortothe'helix. Care must be taken to prevent any secondary electrons from escaping from the collector and impinging upon the helix.

Space charge repulsion can also be seriously harmful in depressed potential collector operation. In the traveling wave tube where an electron beam is magnetically focused through a long tube, as the potential of the collector is reduced the electronflow becomes unstable, and most of the electrons are turned back. This effect is due to the negative space charge of the electrons which depress the potential at the center of the beam." The collector must therefore be designed so that no instabilities will occur at the depressed collector voltages. In this connection it should be noted that the closer the beam is to the electrode surface the niore the potential of the electrode may be depressed before the beam becomes unstable. i i i It is therefore an object of my iu'vention to provide an improved electron collector for efficiently collecting a multi-velocity beam of electrons, i.'e., one having a distribution of electron velocities. T

Another object of my invention is the provision of an improved plural-stage electron collector'wherein the undesirable effects of depressed'potential collection of a multi-velocity beam of electrons are minimized.

Briefly, according to my invention a plural-stage electron collector comprises a preferably cup-shaped elongated hollow conductive member axially segmented and with the segments insulated from each other. To prevent excessive escape of secondary electrons, the cup is fabricated with'a magnetic section at its closed end and a non-magnetic section at its open end to shape the magnetic focusing field of the tube so that no electrons are collected nearthe open end of the cup. To further inhibit secondary. electron escape the inside cylindrical wall of the cup .is coated with a material of low secondary emission yield, is, grooved, and is made convergent toward the open end. V i

zwidth toward the end wall -taper. The overall length of Ll-pin h s; the pp p 3 .lnthedrawingsz Fig. 1 is a longitudinal section view of the collector end of a traveling wave tube illustrating a segmented electron collector according to my invention; and

Fig.2 is atransverse sectionfviewflaken along line 2-2ofFig.l.

In the drawings, an electron collector is shown sealed to theend of an envelope 14 of a conventional traveling wave tube by an expansion-matching ring 16 of 'a material such as a Kovar alloy. The endjofa conventional helix 18 is shown mounted within the envelope 14. The electron collector 10 comprises an elongated, longitudinally-segmented cup and includes an open-ended front segment 22 and a closed-ended back segment 26 sealed together in end-'to-end relation witha separating ceramic ring 30. Heatradiatingstructures 3 8 and 42 are mounted respectively about the front segment 22 and the back segment 26. The two radiating structures comprise respectively inner cylinders 46 and 50,:outer cylinders 54 and 58, and a plurality of longitudinally extending radial vanes 62 and 66 mounted therebetween.

- The inside longitudinal wall of the electron collector 10, including the front segment 22 and the back segment 26, is convergent toward its open end and is provided with a plurality of longitudinal grooves 70 equally spaced around the periphery thereof. The front segment 22 is itself segmented into two parts. An'open-ended front part 74 is composed of an electrically conductive, nonmagnetic material, such as copper. A back part .78 sealed to the ceramic ring 30 is composed of an electrically con- 22, 0.4 inch; the back part 78 of the front segment 22, 0.3

inch; the thickness of the ceramic ring 30, 0.05 inch; and

the length of the back segment 26, 1.25 inches. The outside diameter of the front and back segments 22 and 26 is 1.0 inch; the thickness of the end wall 82, 0.1 inch; the thickness of the cylindrical wall of the back segment 26 adjacent the end wall 82, 0.050 inch; and the thickness of the cylindrical wall ofthecopper front part 74 of the front segment 22 near the expansion matching ring 34, 0.4 inch. Such a preferred embodiment'has been operated'with, for example, potentials of 4400 voltson the helix 1812300 volts on the front segment 22; and 1450 volts on the back segment 26. With these conditions an overall efficiency er 57% was measured. The effiductive, magnetic material such'as cold, rolled steel.

Likewise, the back segment 26 of the collector 10 is composed of an electrically conductive, magnetic material. The end wall 82 of the back segment 26 of the electron collector 10 is provided with a plurality of grooves 86 which may, for example, be in the form of concentric circles or chordal lines as shown. All surfaces inside the collector cup are coated with carbon in order to minimize the secondary electron emission.

A solenoid 90 coaxially disposed with the electron collector 10 extendsalong the traveling wave tube and approximately halfway along the electron collector 10. The solenoid 90 serves to provide an axial magnetic focusing field to prevent beam divergence as it traverses the helix 18 and also to provide a divergent magnetic field the electron collector 10. The divergent field is illustrated by the dotted lines 94 of .Fig'. l.' As will be appreciated from such illustration, the'copper front part 74 of the front segment magnetic field parallel for a distance part way through the front segment 22. Divergence of the magnetic field begins only after the magnetic lines of force reach the magnetic back part 78. Thus, low velocity electrons, which are collected by the front segment 22, impinge mostly on the back part 78 thereof. lBy maintaining low velocity electron impingement onthe back part 78 of the front segment 22, escape of secondary electrons from the open end of the collector 10 is greatly minimized.

in accordance with the theory of electron collection and depressed voltage collectors as hereinbefore discussed, the higher velocity electrons pass beyond the ceramic ring 30 and are collected by the back segment 26 which is operated at a potential depressed below the potential on the front segment 22. i i

According to a' preferred embodiment construction of my invention incorporated in a 100 watt traveling wave tube, the inside cylindrical wall of the electron'collector 110 is provided with a convergent taper of approximately 310. The grooves 70 in the inside cylindrical wall are 0.045 inch deep and 0.040 inch wide. The lands between :the grooves 7 are 0.040 inch wide at. the small diameter end of the inside cylindrical wall and" taper to a greater 82 corresponding to the wall the electron collector 10 is front part 74 of thelfrpntsegment 22 serves to maintain the ciency measurement of a prior art single stage collector with corresponding helix current was measured at only What is claimed:

1. An electron collector for collecting a beam v of electrons comprising an elongated hollow member including a front segment and a back segment disposed coaxially end-to-end and insulatively mounted to each other, the inside wall of said hollow member being convergent toward and openingat the front end thereof, said front segment including a first portion at the front end thereof and a second portion adjacent said back seg ment, said first portion consisting of an electrically conductive non-magnetic material and said second portion and said back segment both consisting of anelectrically conductive magnetic material. V I

2. An electron collector for collecting .a beam of electrons comprising an elongated hollow member including a front segment and a back segment disposed coaxially end-to-end and insulatively mounted to eachother, the inside wall of said hollow member. being convergent toward and opening at the front end thereof, said front segment including a first portionat thev frontend thereof and a second portion adjacent said .back segment, said first portion consisting of an electrically conductive. nonmagnetic material and said second portion and said back segment both consisting ofan electrically conductive'magnetic material, and means for providing .a magnetic field extending into the open end of said hollow member, said magneticfield being substantially axially parallel to substantially the far end of said firstportion and then being divergent. V 7 i 3. An electron collector for collecting. a beam of electrons comprising an elongated .hollowmember including a front segment and a back segment disposed coaxially end-to-end and insulatively mounted to each other, the inside wall of said hollow member being grooved and convergent toward and opening at the front end thereof, said front segment including a first .portionat. the front end thereof and a second portion adjacent said back segment, said first portion consisting Of, electrically conductive non-magnetic material andsaid second portionand said back segment both consisting of an electrically conductive magnetic material, and means for providing a magnetic field extending into the open end of said hollow memben-said magnetic field being substantially axially parallel to substantially the far end of said first portion and then being divergent.

4. An electron collector for collecting a millti-velocity beam of electrons comprising an elongated cup-like member including. anopen-ended front segment and a closedsecond portion and said back segment both consisting of an electrically conductive magnetic material, and means for providing a magnetic field extending intothe open end of said cup-like member, said magnetic field being substantially axially parallel to substantially the far end of said first portion and then being divergent.

5. An electron collector for collecting a multi-velocity beam of electrons comprising an elongated cup-like member including an open-ended front segment and a closedended back segment disposed coaxially end-to-end and insulatively mounted to each other with an intermediate ceramic ring, the inside longitudinal wall of said cuplike member being longitudinally grooved and convergent toward the open end thereof, at a taper of approximately the inside closed-end wall being grooved along parallel chordal lines, both said longitudinal and said closed-end walls being coated inside with a material of low secondary emission yield, said front segment including a first portion at the front end thereof and a second portion adjacent said back segment, the openings in said first and second portions being of substantially equal axial lengths, said first portion consisting of an electrically conductive non-magnetic material and said second portion and said back segment both consisting of an electrically conductive magnetic material, and solenoid means surrounding and extending substantially along said front segment for providing a magnetic field extending into the open end of said cup-like member, said magnetic field being substantially axially parallel to substantially the far end of said divergent.

first portion and then being 6. In a traveling wave tube in which an electron beam is projected along a delay electrode for interaction with the field of said electrode and thence to a collector electrode; the combination with said delay electrode of a collector electrode comprising an elongated hollow conductive means including a front segment and a back segment disposed in end-to-end relation and insulated with respect to each other and said delay electrode, said front segment including a front portion of non-magnetic material and a back portion of magnetic material, at least References Cited in the file of this patent UNITED STATES PATENTS 2,284,733 Haeif June 2, 1942 2,515,997 Haeif July 18, 1950 2,680,209 Veronda June 1, 1954 2,753,482 Dorgelo July 3, 1956 2,853,641 Webber" Sept. 23, 1958 FOREIGN PATENTS 7 9 G e t t n -,-t--.---.- y 1 91 7 

